U.S. Pat. No. 4,575,195, which is hereby incorporated by reference, discloses a series of endoscope relay optics characterized by the Petzval sum being corrected to a desired degree primarily by index difference, thereby avoiding any spaced high powered elements or sharply bent elements. Spherical aberration is corrected by providing aspheric surfaces on the exposed surfaces of negative plastic elements glued to positive glass elements. The design of the lenses discussed in the patent is very flexible and allows a high degree of correction so that the image quality at the eyepiece of the endoscope is high.
However, quality usually has its price, and the price in this case is a relatively large number of glass-air and plastic-air interfaces for most of the embodiments. Thus, while the lenses themselves are readily manufacturable in the sense of having relatively gentle surfaces, the cost of coating the large number of interfaces can be considerable.